Method for producing shale oil from an oil shale formation

ABSTRACT

A method for producing shale oil from a subterranean oil shale formation by controlled in situ combustion by exploding a relatively high energy device within an oil shale formation thereby forming a fragmented vertically extensive zone of rubble having a substantially void space at the top thereof. The zone is filled with a layer of granular material for permeability adjustment and a controlled combustion front is initiated by injecting a fluid comprising a mixture of oxygen-containing gas and aqueous liquid substantially near the top of the granularfilled fragmented zone so as to advance a combustion front down the zone and produce oil shale pyrolysis products.

United States Patent [72] Inventor Michael Prats Houston, Tex. [21] Appl. No. 768,666 [22] Filed Oct. 13, I968 [45] Patented July 20, 1971 [73] Assignce Shell Oil Company New York, N.Y.

[54] METHOD FOR PRODUCING SHALE OIL FROM AN OIL SHALE FORMATION 11 Claims, 6 Drawing Figs.

[52] US. Cl 166/259, 166/247, 166/257, 166/261, 166/309 [51] Int. Cl ..E21b 43/24, E2 lb 43/26 [50] Field of Search 166/247, 256, 257, 259. 261, 272, 271, 309, 302, 299

[56] References Cited UNITED STATES PATENTS 2,780,449 2/1957 Fisher et al. 166/259 3.113.620 12/1963 Hemminger 166/247 X 3,342,257 9/1967 Jacobs et a1. 166/247 3,460,620 8/1969 Parker t 166/257 3,465,819 9/1969 Dixon 166/247 Primary ExaminerStephen .I. Novosad Attorneys-Louis .l. Bovasso and .l. H. McCarthy thereof. The zone is filled with a layer of granular material for permeability adjustment and a controlled combustion front is initiated by injecting a fluid comprising a mixture of oxygencontaining gas and aqueous liquid substantially near the top of the granular-filled fragmented zone so as to advance a combustion front down the zone and produce oil shale pyrolysis products.

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M. PRATS e. Merl A4 ms ATYTORNEY METHOD IFOR PRODUCING SHALE OIL FROM AN OIL SHALE FORMATION BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to an improved in situ combustion method of producing shale oil from a subterranean oil shale ormation by normalizing the permeabilities of the flow paths between the rubblized shale fragments in the fragmented formation under controlled combustion temperature conditions.

2. Description of the Prior Art The use of contained nuclear explosions has been proposed in subterranean oil shale formations in an attempt to break up the oil shale formation so that shale oil can be recovered from the rubbled zone by known techniques, such as in situ retortmg.

Experience has shown that when a relatively high-energy device, such as a nuclear bomb, is exploded within a subterranean earth formation, an almost spherical cavity filled with hot gases is formed. This cavity expands until the pressure within the cavity equals that of the overburden. On cooling, the roof of the cavity collapses since, generally, it cannot support itself, and a so-called chimney develops. Chimney growth ceases when the rock pile substantially fills the cavity, or, stable arch develops. In both cases, a substantially void space is formed below the overburden and above the rubble contained within the chimney. Surrounding the chimney isa fractured zone which results from the shock of the nuclear ex plosion.

A zone or chimney of fragmented oil shale resembles an oil reservoir in the sense that each is a permeable formation that contains combustible organic material. A combustion front can be advanced through such a formation by injecting an ox ygen-containing fluid such as air. The fragmented oil shale differs from an oil reservoir in having no matrix permeability and having relatively high, and widely varying, permeabilities in the channels that are formed by the interstices between the rock fragments.

In an oil reservoir in which the rock matrix is permeable, an efficient production of oil can often be attained by a dry, forward combustion process in which the combustion-supporting fluid is a gas such as air, and the combustion front is advanced in the direction of the air flow. The combustion generates hot combustion products that heat the oil, reduce its viscosity and displace most of it ahead of the combustion front. In some oil reservoirs, some advantage is provided by injecting a combustion-supporting fluid that contains an aqueous liquid; but such a use of an aqueous liquid is not essential, and it is avoided wherever its use involved problems such as gravity layover of the combustion front, damage to the reservoir formation, etc.

A subsurface detonation of a relatively high-energy explosive device, such as a nuclear bomb, is a desirable method of forming a vertically extensive zone of fragmented oil shale or rubble within a subterranean oil shale formation. As discussed hereinable, the underground nuclear detonation tends to form a chimney of rubble, i.e., a vertically extensive cylindrically shaped zone having a void space at the top. It has been suggested, as for example, in a U.S. Pat. to Hemminger, No. 3,1 13,620, to produce shale oil from a chimney of rubble by igniting the hydrocarbons at the top of the fragmented zone and injecting a combustion-supporting fluid near the top of the fragmented zone and advancing a combustion front down through the zone while producing oil shale pyrolysis products from near the bottom of the zone. In such a combustion process, in comparison with a combustion drive in an oil reservoir, the proportion of organic material remaining in the combustion zone is greater than in an oil reservoir while the heat capacity of the rocks in the zone is smaller. The presence of the greater proportion of organic material increases the proportion of combustion-supporting fluid that tends to be consumed in generating unnecessary heat. As soon as a portion of oil shale has been pyrolyzed, any additional heating of the residue is unnecessary. The proportion of organic material that is left by the pyrolysis of an oil shale is more than is needed to provide the heat used in pyroilyzing the oil shale.

ln advancing an underground combustion front, the genera tion of unnecessary heat may be avoided by incorporating aqueous liquid in the combustion-supporting fluid. The aqueous liquid reduces the temperature and rate of fuel consumption in the combustion zone. Because of the relatively low heat capacity in a fragmented oil shale, the water-to-air rates of such a combustion-supporting fluid is relatively critical and amount to about three-fourths as much as required for a viscous oil reservoir.

SUMMARY OF THE INVENTION It is an object of this invention to provide an improved method for producing shale oil by underground combustion retorting of fragmented oil shale under controlled in situ combustion conditions.

It is a further object of this invention to normalize the effective permeabilities of the interstices between the oil shale fragments within a fragmented zone so as to increase the sweep efficiency of the recovery process.

These objects are carried out by igniting the hydrocarbons present at the top of a chimney or fragmented zone of rubble formed within an oil shale formation. A combustion-supporting fluid comprising a mixture of oxygen-containing gas and aqueous liquid is injected substantially near the top of the fragmented zone thereby advancing a combustion front down the zone. Oil shale pyrolysis products are produced from substantially the bottom of the fragmented zone. A layer of granular material is deposited substantially across an upper portion of the fragmented zone at a location between the depths at which the combustion-supporting fluid is injected and oil shale pyrolysis products are produced. The permeability of the layer is adjusted to a permeability that is substantially uniform across the top of the fragmented zone and is significantly less than the average permeability of the fragmented zone. The combustion front is advanced by injecting the combustion supporting fluid while distributing the aqueous liquid of the mixture substantially across the top of the layer of granular material. Shale oil is then recovered from the oil shale pyrolysis products.

BRIEF DESCRIPTION OF THE DRAWING FIG. I is a vertical cross-sectional view of an oil shale formation prior to detonating a relatively high-energy explosive device within the formation;

FIG. 2 is a vertical cross-sectional view of the oil shale formation of FIG. I after the explosive device has been detonated;

FIG. 3 is a vertical cross-sectional view of the final rubble zone created by detonating the explosive device of FIG. 1;

FIG. 4 is a vertical sectional view of the rubble zone of FIG.

FIG. 5 is a vertical sectional view of an oil recovery process applied to the oil shale formation of FIG. 1; and

FIG. 6 is a vertical sectional view of an alternate oil recovery process applied to the oil shale formation of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows a subterranean oil shale formation 111 having a relatively high-energy explosive device 12 located within the formation Ill. The device 12 may be either nuclear or nonnuclear; if a nuclear device is detonated within the oil shale formation 11, a strong shock wave from the nuclear device begins to move radially outwardly, vaporizing, melting, crushing, cracking and displacing the oil shale formation 11. After the shock wave has passed, the high-pressure vaporized material expands, and a generally spherical cavity 14 (FIG. 2) is formed which continues to grow until the internal pressure is balanced by lithostatic pressure. The cavity 14 persists for a variable time depending on the composition of the oil shale formation 11. The cavity roof 20 then collapses to form a chimney 15 (FIG. 3). Collapse progresses upwardly until the volume initially in cavity 14 is distributed between the fragments of the oil shale of formation 11. The size of the substantially cylindrical rubble zone, i.c., chimney 15 formed by the collapse of the cavity 14, may be estimated from the fact that the initial cavity 14 (FIG. 2) expands until the pressure within the cavity is about equal to the lithostatic pressure.

A zone of permeability 17 within and around the fragmented oil shale formation is formed surrounding the chimney 15 as can be seen in FIG. 3. lfdesired, the permeability of the zone 17 may be increased by surrounding the primary explosive device of FIG. 1 which forms the central cavity with a plurality of devices of lesser explosive energy, subsequently detonated in the manner disclosed in a copending application to Closmann et al., Ser. No. 653,l 39, filed July l3, I967.

A substantially void space 18 is formed at the top of the chimney of rubble 15. When used throughout this specification, the terms fragmented zone" and fragmented zone of rubble refer to the chimney 15 or any other rubbled or fracture-permeated zone formed by the explosion of a relatively high-energy explosive device. In order to normalize the effective permeabilities of chimney 15 in accordance with the teachings of my invention, a layer 22 of granular material (FIG. 4) is deposited across an upper portion of the chimney 15 at a depth between those at which fluids are to be injected and produced from chimney 15 as will be discussed further hereinbelow. This may be accomplished by extending a well 13 into communication with the void space 18 at the top of chimney l and injecting the granular material from an external source (not shown) through well 13 and across, for example, the upper portion of the rubble within chimney 15. The permeability of the layer 22 of granular material is adjusted to one that is uniform across the chimney and is low relative to the average permeability within the chimney 15 in a manner to be discussed further hereinbelow.

Referring now to FIG. 5, a preferred arrangement for producing shale oil from the fragmented zone of FIG. 4, is illustrated. A well 19 is extended to the bottom of the chimney 15 thereby establishing communication with the rubble zone. Well 19 is preferably drilled into the bottom of the rubble 15a within chimney 15 while the rubbled zone is hot, or at least warm, from the aforementioned explosion. Well 19 may be cased at least along the portion traversing the overburden l6 and the formation 11, as at well casing 20, with casing 20 ccmented therein as is well known in the art. Well 19 is equipped with a tubing string 27. A combustion supporting fluid is injected through the annulus formed between tubing string 27 and casing 20 above packer 24 and through openings 25 formed in well 19. The openings 25 may be formed by perforating means well known in the art, as for example, a bullet or jet-type perforator. The fluid then passes into the void space of the upper portion of chimney l5 and down through the layer 22 and into therubble 15a.

The combustion-supporting fluid comprises a mixture of combustion-supporting gas, such as air or oxygen, and aqueous liquid, such as water, and is injected in a manner that distributes the liquid substantially uniformly across the top of the layer 22. Such a distribution of the aqueous liquid may be accomplished by mechanical means well known in the art, such as by fixed or rotating deflectors or jets (not shown) which disperse the water and air mixture, for example, over the surface of the granular layer 22. The injection of air mixed with water that is mechanically distributed throughout the areal extent of the void space 18 within chimney 15 limits the combustion temperature and oxygen consumption of the combustion front. Because of the relatively high proportion of organic fuel that may remain in chimney 15 after the organic components have been pyrolyzed, the injected air must be mixed with enough water to limit the amount of oxygen consumption.

Alternatively, water and air, again for example, may be injected over layer 22 in the form ofa foam. For example, such a foam may be preferably formed by dispersing about 0.2 percent by weight of a foaming agent in water and dispersing the air in the aqueous phase in a proportion of about 2 cubic feet of aqueous phase per 1000 standard cubic feet of air.

A suitable foaming agent is Adofoam," a liquid mixture of anionic surface-active agents, manufactured by Conoco Petrochemicals of Houston, Texas. Nonionic surfactants and mixtures of anionic-nonionic surfactants may also be used.

As the combustion front advances down the chimney 15, oil shale pyrolysis products are recovered at the bottom of well 19. These products pass up through tubing string 27 where the shale oil and gas entrained in the recovered products passes through a heat exchanger 28 and into a separator 29. At this point, the shale oil and gas components are separated as is well known in the art. The recovered combustion-supporting gas may be recirculated from separator 29 through pump 26 and heater 31 as is also well known in the art.

Referring now to FIG. 6, wherein like numerals refer to like parts of FIG. 5, an alternate process for recovering shale oil from chimney 15 is shown. In place of the same well being opened into both the void space 18 and the bottom of chimney IS, a well 30, independent of well 19, is opened into the lower end or bottom of chimney 15. Thus, the circulating combustion-supporting gas is injected through tubing string 27 and onto the granular layer 22 disposed across the upper portion of the rubble 15a within chimney 15. As the combustion front moves downwardly in the manner discussed hereinabove, oil shale pyrolysis products are recovered up well 30 at its lower and adjacent the bottom of rubble 15a. Again, conventional equipment and techniques, such as heater 31, pump 26, separator 29 and heat exchanger 28, may be used for pressurizing, heating, injecting, producing and separating componcnts of the oil shale-pyrolysis products circulating out of the chimney 15.

In a rubble-filled nuclear chimney such as chimney 15, the interstices between the fragments of oil shale have widely different effective permeabilities. A fluid-filled void, such as void space 18, usually exists above the rubble l5 and tends to create an essentially constant pressure differential between any portion of the upper part of the fragmented zone (i.e., the chimney of rubble) and the point at which fluid is withdrawn from near the bottom of the chimney. Fluid injected into the upper portion and withdrawn from near the bottom of the chimney tends to finger through the fragmented oil shale by flowing rapidly through some portions and bypassing others. Such an injection of combustion-supporting fluid causes a combustion front to advance rapidly along some highly permeable channels and bypass other less permeable portions of the fragmented zone. During such a nonuniform advance, conductively transported heat causes the pyrolysis of the oil shale in the bypassed portions, but the pyrolysis products are burned as they flow into zones in which combustion is occurring. This results in an inefficient process that tends to consume much air and organic material while producing relatively little shale oil. The permeability-reducing granular layer 22, deposited at the top of rubble 15a as disclosed hereinabove, materially reduces such fingering or bypassing tendencies. For example, where the interstices within the rubble 15a have relative permeabilities that range from one to 100, the relative velocities of the flows within the interstices tend to cover a similar range. If the permeability-reducing granular layer 22 has a uniform relatively low permeability, the variations between the flow velocities are relatively small; for example, if the granular layer 22 has a permeability of 10, it reduces the range of the relative permeability from one to 10. In general, a granular layer that causes at least a fivefold reduction in the permeability of the fragmented zone limits the velocity variation to less than about 20 percent and provides a significant improvement in the efficiency of the shale oil production.

The granular layer 22 may be deposited within chimney 15 by gravel-packing techniques known to those skilled in the art .the gravel packing. The extent to which the permeability is reduced by a particular granular layer may be determined by determining the pressure drop between the points at which fluid is injected and produced.

If desired, the volume of the permeable zone may be increased'by opening and/or enlarging perforations or fractures into the oil shale in and around the fractured zone 17 surrounding the void space 18 and circulating fluid between the void space 18 and the rubble a resulting from the detonation including flow through a flow path enelusive of such openings prior to injecting the combustion-supporting fluid. The tendency for the inflowing fluid to flow laterally outward into such openings before or during a downward flow through rubble 15a toward the location from which the outflowing fluid is produced in enhanced by the flow resistance imparted by layer 22. a

I claim as my invention: 1. In a method for producing shale oil from a subterranean oil shale formation comprising the steps of:

placing a relatively high-energy explosive device within the formation; exploding the relatively high-energy explosive device within the formation thereby forming a cavity within the formation having a roof beneath the overburden which subsequently collapses to form a fragmented zone of rubble within the oil shale formation and extends fractures through the oil shale formation, the fragmented zone having a substantially void space formed adjacent the top thereof; depositing a layer of granular material of a preselected granular size substantially across an upper portion of said fragmented zone at a location between the depths at which a combustion-supporting fluid is to be injected and oil shale pyrolysisproducts are to be produced; and adjusting the permeability of said layer to a permeability that is substantially uniform across the top of the fragmented zone and is significantly less than the average permeability of said fragmented zone. 2. The method of claim 1 including the steps of: igniting the hydrocarbons present in the formation adjacent the top of said fragmented zone; injecting a combustion-supporting fluid comprising a mixture of oxygen-containing gas and aqueous liquid substantially near the top of said fragmented zone thereby advancing a combustion front down said fragmented zone from the top to the bottom thereof;

combustion-supporting fluid includes injecting air mixed with sufficient water to limit the amount of oxygen of said combustion front.

5. The method of claim 2 wherein the step of injecting a combustion-supporting fluid includes injecting said mixture in the form of a foam comprising 02 percent by weight of a foaming agent in the water and the oxygen-containing gas is dispersed in the aqueous phase in a proportion of approximately 2 cubic feet of aqueous phase per [000 standard cubic feet of air.

6. The method of claim 1 including the step of repeating the step of depositing said layer prior to producing said oil shale pyrolysis products.

I 7. The method of claim 1 including the step of repeating the step of depositing said layer while producing said oil shale pyrolysis products.

8. The method of claim 1 wherein the step of depositing said layer includes depositing granular materials graded into a sequence ranging from coarse to fine in moving upwardly through said layer.

9. The method of claim 1 wherein the step of depositing said layer includes depositing said layer prior to igniting said hydrocarbons.

10. The method of claim 1 including the step of extending at least one well through said formation and into communication with both said void space and substantially the bottom of said fragmented zone;

packing off the communication in said well formed between said void space and the bottom of said-fragmented zone; and subsequently injecting said combustion-supporting fluid down said well and through said layer and producing said oil shale pyrolysis products from the bottom of said fragmented zone up said well. 11. The method of claim 1 including the step of extending at least a first well through said formation and into communication with said void space and subsequently injecting said combustion-supporting fluid down said first well and through said layer; and

extending at least a second well through said formation and into communication with the bottom of said fragmented zone and subsequently producing said oil shale pyrolysis products from the bottom of said fragmented zone and up said well. 

2. The method of claim 1 including the steps of: igniting the hydrocarbons present in the formation adjacent the top of said fragmented zone; injecting a combustion-supporting fluid comprising a mixture of oxygen-containing gas and aqueous liquid substantially near the top of said fragmented zone thereby advancing a combustion front down said fragmented zone from the top to the bottom thereof; producing oil shale pyrolysis products from substantially the bottom of said fragmented zone; and advancing said combustion front by injecting said combustion-supporting fluid while distributing the aqueous liquid of said mixture substantially uniformly across the top of the layer of granular material.
 3. The method of claim 2 including the step of recovering shale oil from said oil shale pyrolysis products.
 4. The method of claim 2 wherein the step of injecting a combustion-supporting fluid includes injecting air mixed with sufficient water to limit the amount of oxygen of said combustion front.
 5. The method of claim 2 wherein the step of injecting a combustion-supporting fluid includes injecting said mixture in the form of a foam comprising 0.2 percent by weight of a foaming agent in the water and the oxygen-containing gas is dispersed in the aqueous phase in a proportion of approximately 2 cubic feet of aqueous phase per 1000 standard cubic feet of air.
 6. The method of claim 1 including the step of repeating the step of depositing said layer prior to producing said oil shale pyrolysis products.
 7. The method of claim 1 including the step of repeating the step of depositing said layer while producing said oil shale pyrolysis products.
 8. The method of claim 1 wherein the step of depositing said layer includes depositing granular materials graded into a sequence ranging from coarse to fine in moving upwardly through said layer.
 9. The method of claim 1 wherein the step of depositing said layer includes depositing said layer prior to igniting said hydrocarbons.
 10. The method of claim 1 including the step of extending at least one well through said formation and into communication with both said void space and substantially the bottom of said fragmented zone; packing off the communication in said well formed between said void space and the bottom of said fragmented zone; and subsequently injecting said combustion-supporting fluid down said well and through said layer and producing said oil shale pyrolysis products from the bottom of said fragmented zone up said well.
 11. The method of claim 1 including the step of extending at least a first well through said formation and into communication with said void space and subsequently injecting said combustion-supporting fluid down said first well and through said layer; and extending at least a second well through said formation and into communication with the bottom of said fragmented zone and subsequently producing said oil shale pyrolysis products from the bottom of said fragmented zone and up said well. 